Delta-sigma modulation is a method for converting analog signals into digital signals. The modulation is done using error feedback through a feedback loop in which the difference between the input (analog) signal and the output (digital) signal is measured using a digital-to-analog converter in the feedback loop. The measured difference is used to improve the conversion. Delta-sigma modulation has found increasing use in modern electronic components, such as mobile telecommunications equipment and audio equipment.
FIG. 1 shows an example of a delta-sigma modulator 10, as known in the art. The delta-sigma modulator 10 has a radio frequency (RF) input 20 and, in this example, has three stages, i.e. is a third order delta-sigma modulator. The number of stages can vary depending on the frequency of the input signal at the RF input 20 and the required accuracy of the output signal at the digital output 140. In FIG. 1, each one of the three stages comprises a resonator 40, 60 and 80, a transconductance element 35, 55 and 75 as well as a digital-to-analog converter 110, 112, 114.
The resonators are shown here as a first resonator 40, a second resonator 60, and a third resonator 80. The RF input 20 is connected to the first transconductance element 35. The output of the first resonator 40 is added at a node 30 to the output of a first digital-to-analog converter 114 and the output of the first transconductance element 35. The sum of the currents at the first node 30 is thus the sum of the currents of the output of the first transconductance element 35, the first resonator 40 and the output of the first digital-to-analog converter 114. The first node 30 is connected to the input of the second transconductance element 55.
Similarly, a second node 50 is connected to the second resonator 60, the output of the second digital-to-analog converter 112 and the output of the second transconductance element 55. The second node 50 forms the input to the third transconductance element 75. Finally the third resonator 70, the output of the third digital-to-analog converter 110 and the output of the third transconductance element 75 are connected at a third node 70. The third node 70 forms the input to a quantizer 90 that is typically a flash ADC.
The first digital-to-analog converter 114, the second digital-to-analog converter 112, and the third digital-to-analog converter 110 each have their inputs connected to a feedback loop 120. As described above, the feedback loop 120 is used to provide an error feedback signal and thus improve the accuracy of the conversion of the analog RF input signal received at the RF input 20 to the digital output signal at the digital output 150.
FIG. 2 shows an example of a prior art digital-to-analog converter 200 which is used in the circuit of FIG. 1 as the digital-analog converters 112, 114, 116. The digital-analog converter 200 of FIG. 2 comprises a plurality of cells 201-1, 201-2 . . . , 201-n that are connected to a differential output 255. For simplicity, the circuit is only shown for one of the cells 201-1, 201-2 . . . , 201-n. FIG. 2 shows, however, that there are outputs for each of the cells 201-1, 201-2 . . . , 201-n which are then added together to form the differential output 255.
The illustrated cell 201-1 shows a data terminal 205 and an inverted data terminal 210 which receive data from the digital output 150 of delta-sigma modulator 10. The data terminal 205 is connected to a gate 215g of a first field effect transistor (FET) 215. The inverted data terminal 210 is connected to a gate 220g of a second FET 220. Source 215s of the first FET 215 and source 220s of the second FET 220 are commonly connected to a drain 235d of a third FET 235 and through the source 235s of the third FET 235 to a resistor 240 and subsequently to ground 245. The third FET 235 and the resistor 240 form a current source through which current is flowing. This flowing current leads to noise in the analog-digital converter 200. There may also be a mismatch in the channel (215s-215d) of the first FET 215 and the channel (220s-220d) of the second FET 200 due to process variations that can affect the operation of the digital-analog converter 200.
Gate 235g of the third FET 235 is connected to a bias 230. The drain 220d of the second FET 220 is connected to a first terminal 255-1 of the differential output 255, and the drain 215d of the first FET 215 is connected to a second terminal 255-2 of the differential output 255.
The digital-analog converter 200 has also a load circuit 260 connected across the differential output 255. The load circuit 260 comprises a first capacitor 265 with one side connected to ground 270, and a second capacitor 275 with one side connected to the ground 270. The other side of the first capacitor 265 and the other side of the second capacitor 275 are connected to through an inductor 285 to a supply voltage Vcc 280.
The output signal is formed by the current multiplied by duration of the bits at the data inputs 205 and 210
The digital-analog converter 200 shown in FIG. 2 is DC-coupled and permanently injects noise into the load in the current source including the resistor 240 that is then fed back into the delta-sigma modulator 10 through the feedback loop 120. The output current of the digital-analog converter 200 depends non-linearly on the drain voltage of the first FET 205 and the second FET 220 that can cause intermodulation due to difference in the channels of the first FET 205 and the second FET 220.
There may also be mismatch between the cells 201-1, 201-2 . . . , 201-n of the digital-analog converter 200 because of manufacturing tolerances. This mismatch may lead to non-linearity of the digital-analog converter 200. In particular it will be noted that the third FET 235 which forms part of the current source may have a size of a few square micrometers and thus changes in size due to processing issues can be significant.
One known solution for avoiding the injection of noise into the load is to use an AC-coupled feedback path, as disclosed in United States Patent Application Publication No. US 2008/0062022 (Melanson) which discloses a delta-sigma modulator having an AC-coupled feedback path to reduce signal level in the loop filter. The delta-sigma modulator of the Melanson application has at least to feedback paths corresponding to integrators. In the disclosure of Melanson, one of the feedback paths from the quantizer output is DC-coupled, and another one of the feedback path is AC-coupled.